Pluripotent stem cells such as ES cells (embryonic stem cells) or iPS cells (induced pluripotent stem cells) require such technique, for example, that they are to be co-cultured with feeder cells, to be cultured in a conditioned medium (CM) derived from feeder cells, or the addition of basic FGF (bFGF/FGF2, basic fibroblast growth factor) or LIF (leukemia inhibitory factor), or the like, in order to maintain pluripotency thereof. Otherwise, they will lose pluripotency caused by environment and condition of cells, and they may be sometimes easily differentiated. Therefore, it is important to know exactly an undifferentiated state (a state having pluripotency) or the differentiated state of the stem cells.
In the field of regenerative medicines, the pluripotent stem cells are transplanted after being differentiated into cells of interest. However, when the undifferentiated cells are mixed in the cells to be transplanted, there is risk that they may cause oncogenesis. Therefore, technique of determining whether the undifferentiated stem cells are mixed in the pluripotent stem cells that have been subjected to differentiation-inducing treatment is important.
However, it is difficult to determine the differentiated state of the cells by cell appearance.
Therefore, as a method for determining the differentiated state of the pluripotent stem cells, a method for detecting markers which indicates the differentiated state has been carried out. For example, as pluripotency markers of the pluripotent stem cells, an alkaline phosphatase, Nanog, Oct4, TRA-1-60, Sox, LIF-R, and the like have been known. Since the pluripotent stem cells express these markers in the undifferentiated state, it is capable of determining whether the stem cells have maintained the undifferentiated state by detection of these markers. However, these markers are not capable of determining a differentiated state unless the stem cells are differentiated to a certain degree (for example, to any of three germ layers). Therefore, it is difficult to determine at an early stage of differentiation after induction of differentiation, whether the undifferentiated cells maintain the undifferentiated state (pluripotency), or whether it is in a state where differentiation potential to differentiate in the future has been acquired.
On the other hand, a group of transcription factors with a fork head or a winged-helix DNA-binding domain which is referred to as Fox (Forkhead box) has been known (Non-Patent Literature 1), and about 50 kinds are known to be present in human (FOXB1, FOXB2, or the like). Although a protein expressed by Fox has been known to function mainly as a development regulating factor, a tissue-specific regulating factor, and a cell cycle regulating factor, there are also ones whose action is little clarified.